Copyright © 2009 Pearson Education, Inc. PowerPoint Lectures for Biology: Concepts & Connections, Sixth Edition Campbell, Reece, Taylor, Simon, and Dickey Chapter 5 The Working Cell Lecture by Richard L. Myers

Introduction: Turning on the Lights to Be Invisible  Some organisms use energy-converting reactions to produce light –Examples are organisms that live in the ocean and use light to hide themselves from predators  Energy conversion involves not only energy but also membranes and enzymes  So, production of light involves all of the topics covered in this chapter Copyright © 2009 Pearson Education, Inc.

MEMBRANE STRUCTURE AND FUNCTION Copyright © 2009 Pearson Education, Inc.

5.1 Membranes are a fluid mosaic of phospholipids and proteins  Membranes are composed of phospholipids and proteins –Membranes are commonly described as a fluid mosaic –This means that the surface appears mosaic because of the proteins embedded in the phospholipids and fluid because the proteins can drift about in the phospholipids Copyright © 2009 Pearson Education, Inc.

Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein

5.1 Membranes are a fluid mosaic of phospholipids and proteins  Many phospholipids are made from unsaturated fatty acids that have kinks in their tails –This prevents them from packing tightly together, which keeps them liquid –This is aided by cholesterol wedged into the bilayer to help keep it liquid at lower temperatures Copyright © 2009 Pearson Education, Inc.

Hydrophilic head WATER Hydrophobic tail WATER

5.1 Membranes are a fluid mosaic of phospholipids and proteins  Membranes contain integrins, which give the membrane a stronger framework –Integrins attach to the extracellular matrix on the outside of the cell as well as span the membrane to attach to the cytoskeleton Copyright © 2009 Pearson Education, Inc.

Cholesterol Glycoprotein Glycolipid Carbohydrate of glycoprotein Phospholipid Microfilaments of cytoskeleton Integrin

5.1 Membranes are a fluid mosaic of phospholipids and proteins  Some glycoproteins in the membrane serve as identification tags that are specifically recognized by membrane proteins of other cells –For example, cell-cell recognition enables cells of the immune system to recognize and reject foreign cells, such as infectious bacteria –Carbohydrates that are part of the extracellular matrix are significantly involved in cell-cell recognition Copyright © 2009 Pearson Education, Inc.

5.1 Membranes are a fluid mosaic of phospholipids and proteins  Many membrane proteins function as enzymes, others in signal transduction, while others are important in transport –Because membranes allow some substances to cross or be transported more easily than others, they exhibit selectively permeability –Nonpolar molecules (carbon dioxide and oxygen) cross easily –Polar molecules (glucose and other sugars) do not cross easily Copyright © 2009 Pearson Education, Inc. Animation: Overview of Cell Signaling Animation: Signal Transduction Pathways

Enzymes

Messenger molecule Activated molecule Receptor

5.2 EVOLUTION CONNECTION: Membranes form spontaneously, a critical step in the origin of life  Phospholipids, the key component of biological membranes, spontaneously assemble into simple membranes –Formation of a membrane that encloses collections of molecules necessary for life was a critical step in evolution Copyright © 2009 Pearson Education, Inc.

Water-filled bubble made of phospholipids

Water

5.3 Passive transport is diffusion across a membrane with no energy investment  Diffusion is a process in which particles spread out evenly in an available space –Particles move from an area of more concentrated particles to an area where they are less concentrated –This means that particles diffuse down their concentration gradient –Eventually, the particles reach equilibrium where the concentration of particles is the same throughout Copyright © 2009 Pearson Education, Inc.

5.3 Passive transport is diffusion across a membrane with no energy investment  Diffusion across a cell membrane does not require energy, so it is called passive transport –The concentration gradient itself represents potential energy for diffusion Copyright © 2009 Pearson Education, Inc. Animation: Membrane Selectivity Animation: Diffusion

Molecules of dye MembraneEquilibrium

Two different substances MembraneEquilibrium

5.4 Osmosis is the diffusion of water across a membrane  It is crucial for cells that water moves across their membrane –Water moves across membranes in response to solute concentration inside and outside of the cell by a process called osmosis –Osmosis will move water across a membrane down its concentration gradient until the concentration of solute is equal on both sides of the membrane Copyright © 2009 Pearson Education, Inc. Animation: Osmosis

Selectively permeable membrane Solute molecule Lower concentration of solute H2OH2O Solute molecule with cluster of water molecules Net flow of water Water molecule Equal concentration of solute Higher concentration of solute

5.5 Water balance between cells and their surroundings is crucial to organisms  Tonicity is a term that describes the ability of a solution to cause a cell to gain or lose water –Tonicity is dependent on the concentration of a nonpenetrating solute on both sides of the membrane –Isotonic indicates that the concentration of a solute is the same on both sides –Hypertonic indicates that the concentration of solute is higher outside the cell –Hypotonic indicates a higher concentration of solute inside the cell Copyright © 2009 Pearson Education, Inc.

5.5 Water balance between cells and their surroundings is crucial to organisms  Many organisms are able to maintain water balance within their cells by a process called osmoregulation –This process prevents excessive uptake or excessive loss of water –Plant, prokaryotic, and fungal cells have different issues with osmoregulation because of their cell walls Copyright © 2009 Pearson Education, Inc. Video: Paramecium Vacuole Video: Chlamydomonas Video: Turgid Elodea Video: Plasmolysis

Isotonic solution (B) Lysed(C) Shriveled (D) Flaccid(E) Turgid (F) Shriveled Hypertonic solution Hypotonic solution Plant cell Animal cell (A) Normal Plasma membrane (plasmolyzed)

5.6 Transport proteins may facilitate diffusion across membranes  Many substances that are necessary for viability of the cell do not freely diffuse across the membrane –They require the help of specific transport proteins called aquaporins –These proteins assist in facilitated diffusion, a type of passive transport that does not require energy Copyright © 2009 Pearson Education, Inc.